1. Field of the Invention
The present invention relates to a multi-layer preform for draw-blow forming and a process for the preparation thereof. More particularly, the present invention relates to a preform for the production of a multi-layer polyester drawn bottle excellent in the gas barrier property and a process for the preparation of this preform.
2. Description of the Prior Art
Drawn polyethylene terephthalate (PET) bottles have been used for carbonated refreshing drinks, beer and whiskey in foreign countries and for not only edible liquids such as soy sauce, other sauces, dressings and edible oils but also non-edible products such as synthetic detergents and cosmetics in Japan. Recently, the use of drawn PET bottles for beer, carbonated refreshing drinks and fruit juices is broadened in Japan as well as in foreign countries.
Since drawn PET bottles are excellent in the transparency, strength, light weight characteristic, gas barrier property and gas pressure resistance, the application fields of drawn PET bottles have been broadened. Even though drawn PET bottles are excellent in the various properties among plastic vessels, since drawn PET bottles are composed of plastics after all, they allow permeation of gases though the permeability is low, and they are inferior to metal cans and glass bottles which are completely gas-impermeable. Accordingly, if a carbonated drink is filled in a drawn PET bottle and the bottle is sealed, there is a limit of the storage period and the distribution period is inevitably limited, with the result that the marketing plan is restricted. In this case, the higher is the carbon dioxide gas pressure and the smaller is the volume of the vessel, the severer is the above restriction.
As means for solving the problem of the gas permeability of a drawn PET bottle, there has been proposed a method in which a polyester layer is laminated with a gas-barrier resin such as an ethylene-vinyl alcohol copolymer or a vinylidene chloride resin. However, various technical problems to be solved are included in the practical production of gas-barrier multi-layer PET bottles, as described below.
A bottomed preform for draw-blow forming is ordinarily prepared by injection molding of a resin. When a multi-layer preform is prepared, injection molding should be conducted for the respective resin layers independently. Furthermore, a certain thickness exceeding the critical value is necessary for injection of a resin (about 0.8 mm is necessary in case of an ethylene-vinyl alcohol copolymer; when the viscosity of the injected ethylene-vinyl alcohol copolymer is low and the injected vessel has a shallow bottom, a thickness of 0.8 mm is necessary, and when a long vessel such as a preform is injected-formed, at least 1 mm is necessary). Accordingly, increase of the thickness of the gas-barrier layer cannot be avoided. If the thickness of the gas-barrier resin layer is increased, the manufacturing cost is increased, and bad influences are imposed in the draw-blowing property of the preform and various properties of the formed drawn bottle. The draw-blowing conditions for the production of a multi-layer PET bottle should naturally be determined based on the drawing conditions of the PET layer. Therefore, if the thickness of the gas-barrier resin layer is increased, because of the difference of optimum drawing conditions between the two resins, the drawing property of the preform is reduced and cracking, breaking or peeling is caused in the gas-barrier resin layer. Therefore, such disadvantages as reduction of the gas barrier property and degradation of the appearance characteristic are brought about.
When a gas-barrier resin layer is laminated on a PET layer, in order to eliminate influences of the humidity on the gas-barrier resin layer, prevent the gas-barrier resin layer from falling in contact with the content and also prevent the gas-barrier resin layer from being broken or damaged, it is preferred that the gas-barrier resin layer be arranged as an intermediate layer. In order to form a preform having this three-layer structure, it is necessary to perform the injection operation in three stages, that is, injection of the inner PET layer, injection of the gas-barrier resin layer and injection of the outer PET layer, while exchanging split molds on a core mold. This operation is very troublesome.